class LessThan 
	: public std::binary_function<char const *, char const *, bool>
{
public:
    bool operator () (char const * str1, 
                      char const * str2) const
    {
        return strcmp (str1, str2) < 0;
    }
};

std::map<char*, char*, LessThan> ipmac;

Comparison function for std::map

C++